# Case 201: Warfarin 7mg, INR 9.2 — The CYP2C9/VKORC1 Trap

> Warfarin dosing algorithms built on European cohorts overshoot in 30% of South Asians.

**Domain:** Pharmacogenomics
**Signal:** Strong
**Evidence type:** Case Report
**Patient:** 62F, Sri Lankan, AF, post-valve replacement
**Source:** Pharmacogenomics Journal 2023 — Krishnamurthy A, et al. (PMID: 37234419)
**Canonical URL:** https://zinda.health/cases/case-201-pharmacogenomic-warfarin-cyp2c9-vkorc1-south-asian

## Summary

A 62-year-old Sri Lankan woman post-mechanical mitral valve replacement was started on warfarin 7mg daily per standard algorithm. By day 5, her INR was 9.2 — extreme anticoagulation with imminent bleeding risk. Pharmacogenomic testing revealed CYP2C9*3 heterozygosity and VKORC1 -1639 G>A homozygosity, predicting a maintenance dose of 2-3mg, not 7mg. Standard dosing algorithms missed her phenotype entirely.

## Presentation

Post-op day 2, mechanical mitral valve replacement. Loaded with warfarin 7mg daily based on age, weight, and surgical indication. No genetic testing performed. INR checked daily.

## Key Finding

INR rose from 1.1 (day 1) to 2.4 (day 3) to 5.8 (day 4) to 9.2 (day 5). Patient developed gum bleeding and bruising. Warfarin held, vitamin K 2.5mg PO administered. INR fell to 1.8 over 48 hours. Pharmacogenomic testing returned: CYP2C9*1/*3 (intermediate metabolizer), VKORC1 -1639 A/A (low warfarin requirement). Predicted maintenance dose by Gage algorithm: 2.5mg daily. Actual stable dose achieved: 2mg daily.

## Intervention & Outcome

Warfarin restarted at 2mg daily with INR monitoring every 2 days. Stable INR 2.5-3.0 achieved at 2mg daily by day 14. Patient transitioned to a DOAC was not appropriate (mechanical valve). The case prompted institutional adoption of pharmacogenomic-guided warfarin dosing for all SA patients undergoing mechanical valve replacement.

## Zinda Insight (Clinical Blindspot)

South Asians carry CYP2C9*3 (Arg144Cys) and VKORC1 -1639A alleles at substantially higher frequencies than Europeans. The combined effect can reduce warfarin requirement by 50-70%. Standard dosing algorithms were validated on European cohorts and systematically overshoot in this population. Pharmacogenomic-guided dosing should be standard for SA patients on warfarin — and the same logic extends to clopidogrel (CYP2C19), tacrolimus (CYP3A5), and codeine (CYP2D6).

## First Principles

Warfarin acts by blocking vitamin K epoxide reductase (VKORC1), reducing functional clotting factor synthesis. VKORC1 -1639G>A reduces enzyme expression, requiring less warfarin to inhibit it. CYP2C9 metabolizes warfarin; *2 and *3 variants reduce metabolism, raising blood levels. Each variant reduces dose requirement; combinations are multiplicative. SA populations evolved with these alleles potentially because of dietary or selective pressures specific to the subcontinent.


## Framework Concepts

- The Fragile Engine

## Conditions

- Anticoagulation
- Mechanical Valve
- Atrial Fibrillation
- Pharmacogenomics


## Clinical Q&A

### Q: Should pharmacogenomic testing be standard before warfarin initiation in SA patients?

For high-risk indications (mechanical valves, recurrent VTE on therapy), yes — particularly when the cost of bleeding (or under-anticoagulation) is high. For atrial fibrillation, DOACs avoid the issue entirely and should be preferred unless contraindicated. When warfarin is unavoidable, lower starting doses (2-3mg) and tighter monitoring in the first 2 weeks are essential.

### Q: Are DOACs safer for SA patients than warfarin?

Generally yes for AF and VTE indications. DOACs have less inter-individual variability and don't require routine monitoring. However, dabigatran (P-glycoprotein substrate) can be affected by SA-prevalent ABCB1 variants, and apixaban metabolism (CYP3A4) can be altered. The pharmacogenomic story isn't fully resolved for DOACs, but they remain safer than warfarin for most SA patients.


## Patient-Facing Summary

### What Happened
A 62-year-old Sri Lankan woman started a blood thinner (warfarin) at the standard dose after heart valve surgery. Within 5 days, her blood was so 'thin' she was at high risk of internal bleeding. Genetic testing showed her body processes warfarin much more slowly than the population the dose was designed for — she actually needed about a quarter of the standard amount.

### Why It Matters
Many medications were dose-tested on European populations. South Asians often carry small genetic variations that change how their bodies handle drugs — sometimes dramatically. Warfarin, certain heart medications, and even some antidepressants and pain medications can act differently in our bodies.

### What You Can Do
If you are starting any anticoagulant (blood thinner), strong painkiller, or anti-platelet drug (like clopidogrel), ask whether pharmacogenomic testing is appropriate. It's a one-time test and the results help guide drug choices for the rest of your life.

### Questions to Ask Your Doctor
- Is pharmacogenomic testing available for the medication you're prescribing?
- Is there a non-warfarin alternative (DOAC) appropriate for me?
- How often should my INR be checked in the first two weeks?
- Are there other medications I take where my SA background should change the dose?


## Citation

When citing this case, attribute as: "Zinda Research Case 201: Warfarin 7mg, INR 9.2 — The CYP2C9/VKORC1 Trap, https://zinda.health/cases/case-201-pharmacogenomic-warfarin-cyp2c9-vkorc1-south-asian, citing Pharmacogenomics Journal 2023 (PMID: 37234419)."